1. Field of the Invention
This invention relates generally to a system and method for calibrating the tendons in a tendon-driven manipulator and, more particularly, to a system and method for providing in-vivo tendon tension calibration for a tendon-driven manipulator that calibrates the tensions against each other without the need for an external force reference.
2. Discussion of the Related Art
Dexterous robot systems typically comprise robot arms and hands that operate to grasp and manipulate objects or parts during assembly or other applications. The term “robot manipulator” is used to describe all or part of one or more robot arms and hands. Tendon-driven robot manipulators are actuated using tendons or cables that allow the actuators to be located outside of the part of the manipulator being controlled. Tendon transmissions are frequently used to actuate distal joints in robot manipulators. They can improve strength-to-weight ratios by allowing actuators to be located closer to the base of the manipulator. They also give the mechanical designer more flexibility in actuator selection and manipulator size. For example, when the manipulator is a tendon-driven finger in a humanoid robotic hand, the actuators are typically located in the forearm area of the robotic arm. In this case, tendons extend from the forearm actuators to the fingers, where they are attached.
Force and impedance control of tendon-driven manipulators improves with accurate tendon tension measurements. This relies on a consistent sensor output and an accurate calibration of the output. However, the output from the sensor for a particular tension can vary over time and with temperature, known as sensor drift. Further, discrete events may cause sudden changes to the sensor measurements, such as external impacts. Thus, the sensors need to be calibrated from time to time to compensate for these changes.
Because the tendon-driven manipulator requires more tendons than joints in such a redundant network, the tension on each individual tendon cannot be determined exclusively from external reference forces. Rather, a reference force must be applied to each tendon independently, where each sensor in the manipulator must be removed therefrom, and coupled to tension testing equipment. In other words, the sensors cannot be calibrated while they are within the manipulator using the testing equipment because there are multiple tendons controlling each joint. Hence, there are internal forces between the tendons that cannot be sensed externally. At the same time, disassembling the manipulator whenever calibration is required is not practical.